Question

Funcions of the After Cas9 cleaves the DNA, cellular enzymes will attempt to repair the break. Most of the time, these enzymes repair the DNA without errors. However, Cas9 will keep cutting the DNA at the same location until an error is made. 7. DNA repair errors include losing or inserting random nucleotides at the cut site. Explain how these changes might inactivate a gene. PART 2: Inactivating Genes in Butterflies Robert Reed, a biologist at Cornell University, wanted to identify genes that are important in butterfly wing patterns. He and his colleagues used CRISPR-Cas9 to inactivate different genes, then observed the effects on butterflies. Models of three genes they inactivated are shown in Figure 1. optix gene: spalt gene: HH HHHHH Distal-less gene: HHH Figure 1. Models of three genes involved in butterfly wing patterns: getix segle and Distal-less. The shaded rectangles represent exons, and the horizontal lines yetween the rectangles represent introns. The black triangles above the rectangles point to the target DNA sequertices, which match the guide RNAs made by the scientists. The arrows at the start of each gene represent the transcription start sites. 1. What are exons and introns, and how do they differ? 2. Figure 1 shows that the target DNA sequences are all in exons. Why might scientists want to target sequences in exons, rather than introns, when inactivating genes?

Answer 1

7. Deletion or insertion of the random nucleotide at the cut site might inactivate genes. The inactivation can take place due to several reasons.

  - The addition or deletion of a nucleotide may change the reading frame of genes. Change in the reading frame may create a premature stop codon while transcribing the gene. Premature stop codons will synthesize incomplete inactive proteins.

- The addition or deletion of a nucleotide may affect the site-specificity of the transcription machinery. Most of the machinery proteins involved in the DNA molecular processes are using the sequence specificity for their binding and action on DNA. If the deletion or insertion is affected by the binding site of the transcription machinery, then the gene gets inactivated.

1. Exons are the coding sequences in the DNA. Introns are non-coding regions. Exons are interrupted by introns in their distribution in the genome. Since introns are non-coding regions, they get spliced from the mRNA after transcription.

2. Exons are the coding sequence for the proteins. Targeting exons in gene inactivation will be much easier to achieve inactivation rather than targeting introns. Because introns are not a part of proteins, they have just regulatory functions. Destabilizing the protein is the best way to achieve gene inactivation.